The 5-Step Design Pathway to the Perfect Instrument

There is an array of considerations involved with the development of an ideal surgical device.

Figure 1: Five phases of a user-centered design process. First, learn everything about the user’s needs and human performance requirements to frame a potent creative phase. Then test and evaluate all concepts with users to define the best design strategy to pursue. Finally, conduct iterative prototype testing with end users to optimize the final design. Graphic courtesy of Metaphase Design Group Inc.

Figure 2: A human’s four fingers are effectively hinged joints and do not have significant strength or dexterity moving laterally, whereas the thumb possesses an omnidirectional ball joint support with larger muscles affording greater strength. As the hand closes, it forms a natural grip axis (see arrow) and defines three palmar arches that will shape and inform instrument grips, surface topology, and control interface locations. Image courtesy of Kapandji 1982.

Figure 3: From left to right, bilateral and trilateral precision grips, multilateral power grip, and hook grip. Image courtesy of Metaphase Design Group Inc.

Bryce G. Rutter, Ph.D. , CEO, Metaphase Design08.15.17

The perfect surgical instrument is a seamless extension of the human hand. It makes surgeons better at what they do. It enhances precision, control, and dexterity, while minimizing exposure to stress and strain. The perfect design also makes an important emotional connection with the surgeon. It engages the user on all sensory levels and creates a unique, empowering experience of confident use.

What does it take make a surgical instrument that is the best of the best? Over 30 years designing more than 500 products has helped refine the process to five critical success factors that frame the conditions for powerful product design.

1. Define the Core Team
The perfect instrument begins with a strong, deliberate, and effective core design team. Effective teams are a collaboration of competing and allied interests, with representation from all factions that impact the overall success of the product—engineering, manufacturing, marketing, branding, supply chain, regulatory, human factors expertise, and industrial design. Human factors and industrial design are often sourced outside the company to specific subject matter experts. All these stakeholders at the table means each key decision throughout the project will account for prime business issues that influence ultimate success. Further, each member must have the personality for collaborative, innovative work—open minds, able to keep personal opinions from blocking the exchange of ideas in an open forum, and intrinsically wired with a “glass half full” optimism.

Warning sign the team is impaired? “We can’t do that!” or “We’ve never done that before!” is heard. The appropriate response is, “That’s why we call it a new idea!” In an effective core design team, there is no tolerance for innovation killers—neither people nor points of view. That attitude can harm a project from start to finish. Innovation requires people to think differently, to be comfortable connecting dots that have not been connected before, and to give themselves permission to ask “What if?”

Choosing the team leader is critical. The core team leader does not need a personality that is compatible with everyone on the design team, but an effective core team leader will create a team environment where every member performs at his or her peak. Effective leaders are excellent listeners and create a safe harbor for team members to bring any and all ideas to the table for consideration.

The most important step for the core team leader at the beginning of the design project is to develop a project scorecard that outlines the metrics for decision making that is based on buy-in from all team members. This scorecard will evolve as the project progresses, but its essential role persists—to remove subjectivity and provide a clear, concise framework for decision-making that is on time and on budget.

Most people who have participated in product design projects have observed the following phenomenon. The final product goes to market missing key features or attributes identified during development, with no discernable reason why. A critical role for the core team leader is being a “Design Sherpa,” guiding the project from the start through to commercialization, ensuring all learnings, insights, user and product performance requirements, and sparks of innovation discovered and defined are not lost or dumbed down. The core team leader also manages the inevitable trade-offs and compromises made in product development for the best outcomes, while providing the team members with “information authority”—they have the right to understand the whys and the hows of compromises that affect the integrity of the design.

2. A User-Centered Innovation Strategy
The coolest state-of-the-art technology, materials, or supply chain logistics cannot compensate for a surgical instrument that does not fully address the surgeon’s needs and performance capabilities. A user-centered innovation strategy is anchored in understanding how end users think, feel, and behave; their human performance capabilities and limits; and the task at hand that shapes and informs a highly effective creative phase (Figure 1).

Effective creativity starts with an open mind and a granular exploration of the surgical procedure to illuminate all articulated and unexpressed user needs as well as performance requirements. This mindset rethinks the surgical procedure without automatically inheriting all engineering and design constraints imposed by the design of current instruments. Discovering ergonomic and performance requirements while respecting sensory capabilities will shape and inform concept design strategies that are seamless extensions of the surgeon’s hand. It is key to avoid getting hung up on details; this is the time to stay at the conceptual level, examining different strategic paths for the design to accomplish the overarching goals of the project. This is also the time to engage in lateral thinking—coming at the challenges from every angle and considering all possibilities without latching onto a specific design tactic. The team’s mindset must be open to embrace all possibilities.

Key at this stage is testing the initial design concepts with end users to keep the core design team on target with project goals and block attachment to a concept that will fail. End-user concept testing will instruct the core team on the most effective design strategy to pursue and initiate the tasks of dialing in the details of features and functionality, materials and manufacturing process, and ergonomic and industrial design characteristics. Under the guidance of its leader, the team determines the design direction and advances to where the rubber meets the road; each aspect and characteristic of the product is evaluated in total detail, with each design decision based on whether it adds value or utility, or enhances the human performance of the end user.

The importance and potency of these first two phases of learning and creativity in the user-centered process are far too often underestimated and underfunded. They create the greatest degree of angst not only in team members, but more so those in management who are focused on the end game—they want to see the final design. Giving either phase short shrift, however, results in the development of marginal designs, not world-class, innovative breakthrough designs. When effectively executed, these early stages put in place a foundation for success that allows the core team to move forward assimilating what most do today—iterative building and testing of functional prototypes until the design has been fully optimized and is ready for commercial development.

3. Dissecting & Understanding Users
For the purposes of designing the perfect instrument, a “user” is anyone who touches the product. This includes the surgeon, surgical assistant, and nurses, through to the technicians in central processing placing instruments in sterilization trays for cleaning. Each of these users has unique and specific need states that must be addressed for the product to succeed across the entire “usage ecosystem.”

One axiom in understanding users is what they say they do and how they really behave are never the same. They are not actively misleading the team; rather, they generally lack the intrinsic skill to examine their own behavioral patterns and idiosyncrasies. It’s important not to rely solely on user interviews. By all means, interview every user, but confirm or adjust the behavioral facts by also observing users in action. Users report the emotional and perceptual context within which a new product will be perceived, an aspect of usage just as important as the actual performance of the product in surgery. It isn’t uncommon for designers to include features that have nothing to do with surgical performance but have a significant impact on adoption and evaluation.

To truly understand users, it is best to go to “the point of sweat.” HIPAA has dramatically increased the hurdles for conducting in-surgery research, but it’s still possible to get elbow-to-elbow with surgeons to observe how they use current instrumentation and surgical systems and to study surgical workflow.

The core team leader will need to negotiate the potential user pitfalls that can come from established key opinion leaders (KOL)—or gurus in a particular surgical space. KOLs are often the first resort for evaluating new design ideas, testing prototypes, and gaining access to surgeries for observational research. KOLs bring the value of elite skills and perspectives, but do not discount the lessons to be learned by studying novice users who have not yet mastered surgical techniques and protocols. The challenges novices experience and manifest in instrument precision, control, dexterity, and surgical technique are significantly easier to spot and often illustrate the root cause of human factors affecting the instrument.

Sophisticated surgical procedures make it virtually impossible to observe, take notes, and catch all the ergonomic and behavioral nuances. It is even more difficult to make comparisons between surgeons within the same hospital, or factor in regional or global differences in technique and instrumentation. Video recording surgical procedures is a must. Video provides an irreplaceable post-hoc evaluation tool that enables the behavioral patterns of users to be identified and studied. Video also allows the design team to define unique user personas so the team can clearly delineate product features and benefits, and how they should integrate into a final design for differing global markets and surgical procedure biases.

In most cases, surgeons will be open to talking through what they’re doing and experiencing as surgery unfolds. These narratives are not the whole story, but they provide meaningful insights into surgeon perceptions about the challenges from design features and what would make surgery easier.

Cadaver and live animal research are equally effective in identifying user needs. This research is significantly easier to set up with much less paperwork, and provides unfettered access and discussion with the surgeon and surgical team. For the most part, cadavers and/or live animal anatomies provide an effective human analog for evaluating surgical technique, instrument design, concept testing, and design validation with functional prototypes.

The linchpin for successful user research in live human surgeries as well as in cadaver and live animal labs is that questions must not be leading. For example, asking a surgeon how difficult is it to use a particular instrument already frames the surgeon’s response to be on a scale of difficulty. A more productive technique is posing an open-ended question by asking, “How would you score this instrument on a scale of 1 to 10 from difficult to easy?”

One last caution in user research—while design teams are frequently required to set up surgical test research through sales representatives, it is a good idea for them not to be included in the in-surgery research event. Sales people are hard-wired to overcome objections and user research is very much about finding objections, as well as shortcomings in the instrument being evaluated. The surgical user research will be much more productive if restricted to the surgical, human factors, and design teams.

4. Designing for Human Hands Anthropometric Factors
The first step in human factors engineering is identifying anthropometric requirements that shape and inform the scale and configuration of the instrument design. The variety in human hands is one of the toughest challenges in designing the perfect instrument. A guideline for accommodating the wide disparity in human hand size in the mass market is to use the measurement spread from the 5th female percentile to the 95th male percentile. Designing for this range in hand size means the instrument must accommodate handling that can vary 1.75 inches in length and 0.75 inches in width. Similarly, strength correlates to the cross-section of the muscle, so larger hands with larger muscles are stronger and smaller hands are weaker. For design of controls, connect/disconnect collets and control interface positions the 5th percentile female hand size is the baseline for the “worst case scenario” for reach measurement and strength capacity.

The muscle sets used with an instrument directly influence the instrument dexterity, precision, and control. The relatively smaller and weaker muscles within the hands and fingers provide the highest degree of precision, control, and dexterity, whereas the larger muscles in the forearm and upper arm afford significantly more strength, but far less precision, control, and dexterity. Designing the perfect surgical instrument is all about precision, control, and dexterity, which means the design is all about the human hand. The perfect instrument causes the user to intuitively grasp and manipulate it using the intrinsic muscles of the hand.

There are natural intrinsic geometries in the design of the hand that will also inform instrument design. A natural grip axis presents as a user closes his or her fingers and grasps the instrument, as do three palmar arches that, in turn, position the user’s fingertips in specific regions on the instrument (Figure 2). These natural motions combined with hand size and strength effectively define where fingertip grip surfaces and controls should ideally be located for precision, control, and comfort.

Grip Architectures and Grasping Strategies
Everyone has a smart (dominant) hand and a dumb (non-dominant) hand. On the smart hand, there are three smart fingers and two dumb fingers. The three smart fingers include the thumb with 3 degrees of freedom and the index and middle fingers with 2 degrees of freedom in each joint and minimal lateral control and dexterity. We use these smart fingers in various combinations for trilateral and bilateral dynamic grip architectures in what are called precision grips, and when force is needed using a power grip (Figure 3).

Precision grip architectures allow surgeons to control instruments a fraction of a millimeter whereas power grip architectures allow them to apply significantly greater force at the tool tip. Defining the motions required for the surgical performance in the instrument tooltip is essential to determining the optimal grip architecture and grasping strategy that will allow for the necessary degree of coordination, control, and surgical skill. This is a critical task for the core design team in developing the perfect instrument. Furthermore, human hands have the remarkable capability of sensing differences as small as one to two thousandths of an inch, a perception that can make a profound difference in dexterous control and true surgical performance, so it’s imperative to treat each and every texture in a way that provides the best fingertip traction (Figure 4).

Figure 4: In Metaphase Group’s design of Zimmer’s award-winning Persona knee implant instruments, all fingertip control surfaces were designed to be concave to fit the fingertip anatomy, with a coarse texture orthogonal to the direction of force being applied for maximum traction and to eliminate slippage. Image courtesy of Metaphase Design Group Inc.

Intuitiveness & Cognitive Loading
If the use and operation of an instrument is not explicitly obvious and intuitive, the design is broken. It is the design team’s responsibility to use form, materials, textures, and color to tell the story of how to pick up, push, turn, insert, and use the instrument. Furthermore, if there are cognitive computations required while using the instrument, the surgeon’s mental bandwidth to focus on the task can be dramatically reduced by counterintuitive design.

For example, there is an ongoing debate in the robotic surgical instrument space over reflected motion (e.g., for traditional laparoscopic surgeons, moving the instrument handle down creates a counterintuitive raising of the tooltip) and true motion (e.g., moving the instrument handle down causes the same tooltip motion). For robotic surgical systems that still buy into the traditional notion of providing reflected motion rather than true motion, the surgeon is constantly running three-dimensional computations to compensate for the lack of one-to-one direct motion control. That’s a waste of the surgeon’s cognitive bandwidth.

Weight, Balance, and Form
Weight, balance, and form are often overlooked in instrument design, but cannot be ignored in creating the perfect surgical instrument. These three design factors have a profound effect on surgeon performance. Regardless of the type of instrument, the goal in design is to get the center of mass of the instrument within the compass of the hand, and ideally within the primary grip, whether it be bilateral (finger-thumb) or trilateral (two fingers-thumb) grip. Proper centering of mass reduces and may even eliminate any pendulum effect that amplifies tip movement and control inputs from the surgeon. Directly impacting the center of mass design requirement is “line tug”—the dead weight pull of all suction, irrigation, and power lines running to and from the instrument hand piece that the surgeon must constantly overpower throughout the surgery. Overlooking this design issue is a guaranteed way to build user fatigue into the instrument.

There is no cookie-cutter solution for the optimal primary grip on the surgical instrument, where the index, thumb, and middle finger land on the instrument for the optimal coordination, control, and skill with the tooltip (Figure 5). Depending upon the motions required for the specific surgical technique, the design must sculpt the instrument’s control surfaces where the fingertips land to optimize those motions. For example, in vitreoretinal surgery, where there is a combination of rotary and z-axis motion, the surface topology of the grip needs to allow for an easy twiddle, with flexion and extension of the three smart fingers. Conversely, ENT sagittal, rotary, and reciprocating small bone saws require more of a downward pressure and linear control, which, by design, require different control surface topologies, textures, and locations for the thumb, index, and middle fingers.

Figure 5: In our design of Medtronic’s award winning M5 Microdebrider, we designed ergonomic concave fingertip landing surfaces for the opposing thumb and middle finger to “plug in,” in combination with a non-slip elastomeric “chin” grip for the fourth finger, to allow the index finger to be free to precisely operate the fingertip cutting window control wheel that is also elastomeric to prevent slippage. Image courtesy of Metaphase Design Group Inc.

5. Testing Ideas vs. Concepts vs. Prototypes
The design team seeking to develop the perfect instrument will become very close friends with testing and more testing. Testing ideas, concepts, and prototypes requires incredible, persistent attention to detail to ensure that the results are not an artifact of how the research itself is conducted.

In the early phases of concept exploration, the goal is testing design strategies, not drilling into the specifics of features and benefits. The most effective way to evaluate early-stage concepts is with illustration boards combined with quick 3D sketch models that allow users to knit together the overall big picture of the proposed concept. Typically, surgeons are not effective projecting what an instrument will feel like in surgery while they are sitting at their desks. Whenever possible, the design team should present the selected sketch models in the surgical environment so surgeons can connect the dots on usage. At this stage of testing, design teams frequently discover the most appealing design strategy as well as the key features and attributes that need to be integrated into the next stage of design development.

There’s no way to effectively test a design without getting a model in the user’s hand to simulate usage. For this testing, rapid print models are essential and should be used in combination with picture-perfect renderings of the proposed final design. A proven strategy for design teams assessing the efficacy of a design is to take rapid print models and illustrative renderings into cadaver labs to share with both key opinion leader surgeons and novice users.

Coming out of concept testing, the design team’s focus is on dialing in the details regarding materials, manufacturing processes, and the specifics of visual, haptic, and acoustic feedback. For example, if there is a fingertip control on the instrument, the team needs to dial-in optimal force actuation, the haptic feel of the control, and the acoustical feedback signature when using it.

Testing prototypes provides the last cost effective opportunity in the design process to encapsulate the final design. Functional prototypes do not need to be picture-perfect but must present to the user all functional attributes and feedback that will be embedded in the final design. Testing functional prototypes in cadaver and live animal labs can be an effective tactic in the design process. Prototype testing provides the opportunity to listen to surgeon reactions to the new design, but, more importantly, to observe their functional control over the instrument. Testing in the lab will provide critical direction on how to refine and optimize the ergonomics and industrial design of the instrument.

The Net-Net
The roadmap for designing the perfect instrument never varies, but the pathway has as many different twists and turns as there are design teams and product goals. Inclusive design teams that embrace innovation, reject preconceptions, relentlessly learn about and from their users, remember the intricacies of the human hand, and test relentlessly will find they have produced high-performance instruments that fit the task and the surgeon perfectly.